Neodymium-doped gehlenite crystal and laser using said crystal

ABSTRACT

Neodymium-doped gehlenite crystal and laser using said crystal. The crystal according to the invention has the formula Ca 2-x  Nd x  Al 2+x  Si 1-x  O 7  with 0&lt;×≦1. This crystal can be used as a laser emitter (4) optically pumped by a laser diode (6), whose temperature is not controlled by a Peltier element component.

DESCRIPTION

The present invention relates to crystals and more particularly toneodymium-doped gehlenite monocrystals. It can be used either in thefield of microlasers for integrated optics, optical fibretelecommunications, medicine (microsurgery, skin treatment) and researchon semiconductors, or in the field of power lasers making it possible tocarry out material treatments (welding, perforating, marking, surfacetreatment), photochemical reactions, controlled thermonuclear fusion orthe polarization of the atoms of a gas such as helium. These lasers emitin the infrared between 870 and 1130 nm. More particularly, theinvention applies to lasers optically pumped by a laser diode and whichcan be wavelength tuned between 1050 and 1100 nm.

The most frequently used, commercial solid power lasers make use of agarnet of yttrium and aluminium Y₃ Al₅ O₁₂ (YAG), doped by Nd³⁺.However, this material suffers from disadvantages. Thus, it has acrystallogenesis at a high temperature of 1900° to 2000° C., which makesit difficult and expensive, a segregation of the dopant, narrowabsorption bands, etc. Therefore research has been carried out with aview to finding new matrixes and this has also been made necessary bythe ever improving performance characteristics required for new lasers,namely very high efficiency, miniaturization, emission at variouswavelengths, etc.

Numerous matrixes which can be doped by Nd³⁺ have been proposed in theliterature. Among these matrixes, reference can be made to compoundssuch as melilite ALaGa₃ O₇, gehlenite AGa₂ SiO₇ and akermanite A MGe₂ O,in which A represents the Ca²⁺, Sr²⁺² or Ba²⁺ ions and M representsBe²⁺, Mg²⁺ or Zn²⁺ ions.

Compared with doped aluminium and yttrium garnet, these compounds havethe major advantage of being producible by the Czochralski method, whichis the most widely used in the laser industry and at temperaturesroughly 500° C. lower. In addition, there is no segregation of theluminescent dopant.

Under flash lamp optical pumping, a certain number of these compoundshave the laser effect.

The document A. Keminskii et al. Phys. Star. Sol. (a), 97, 1986, pp279-290, "Crystal structure, absorption, luminescence, properties, andstimulated emission of Ga gehlenite" describes the stimulated emissionin a Ca₂ Ga₂ SiO₇ monocrystal doped with neodymium ions.

The article by W. Ryba-Romanowski et al., J. Phys. Chem. Solids, 50,1989, pp 685-692 "Relaxation of the ³ F_(3/2) level of Nd³⁺ inBaLa_(1-x) Nd_(x) Ga₃ O₇ " describes the laser emission of a monocrystalof BaLa_(1-x) Nd_(x) Ga₃ O₇ with 0≦×≦0.2.

The article by D. J. Horowitz et al, J. Appl. Phys., vol. 43, No. 8,August 1972, "Laser action of Nd³⁺ in a crystal Ba₂ ZnGe₂ O₇, pp3527-3529 describes the laser effect of a Ba₂ ZnGe₂ O₇ crystal dopedwith neodymium with a quantity of 2 mole %.

The article by M. Alan et al., J. Appl. Phys. 39, 1968, "Optical spectraand laser action of neodymium in a crystal Ba₂ MgGe₂ O₇ ", pp 4728-4730describes the laser effect of a crystal of Ba₂ MgGe₂ O₇ doped withneodymium with approximately 2 mole %.

All these compounds have the advantage of a uniaxial quadraticstructure, whereas the YAG crystal is cubic, so that a polarizedemission can be obtained.

Unfortunately, all these compounds suffer from the disadvantage ofcontaining a large amount of gallium, which is expensive. Moreover, itis unstable in oxidation state 3⁺ at high temperature, as a function ofthe working conditions and with the possibility of volatilization.Moreover, due to the volatilization of the gallium, the crystalsobtained by Czochralski growth have inadequate quality characteristicswhen it is a question of obtaining the large dimensions required by thepower laser industry.

Moreover, these compounds have a thermal conductivity making itdifficult to dissipate the heat during their use as a laser emitter in apower laser.

In the field of microlasers, the miniaturization of solid lasers passesthrough the use of laser diodes for optical pumping having much smallerdimensions than those of flash lamps. In addition, laser diodes have ahigh reliability and laser efficiency levels well above those of flashlamps (10 times higher). This high laser efficiency requires a perfectcoincidence between the diode emission line and the absorption bandmaximum relative to the monocrystal.

Due to the wavelength drift of the diode emission during the power riseresulting from the heating of the diode, it is necessary to use a diodetemperature control system with a view to compensating said wavelengthdrift. The diode temperature control is generally brought about by aPeltier element. Unfortunately, the latter has a high electric powerconsumption, which in part destroys the interest of the high diodepumping efficiency.

Only a crystal having wide and intense absorption bands in the diodeemission range making it possible to cover the wavelength drift thereofwould make it possible to get round said Peltier element.

Thus, very recently, a melilite SrGdGa₃ O₇ doped with 2 atomic % by Nd³⁺ions has been proposed for diode pumping (cf. in this connection thedocument by H. R. Verdun et al, Tunable Solid State Lasers IV, 1990, pp405-407, "Growth and characterization of Nd doped aluminates and galateswith the melilite structure").

This material has the advantage of a wide line from approximately 10 nmto 810 nm corresponding to the emission wavelength of the generally usedlaser diodes. However, up to now stimulated emission has not beenobtained with this compound. Moreover, like the preceding compounds, itcontains a large amount of gallium.

The invention also relates to a novel, gallium-free material, which canbe optically pumped by a laser diode without using a Peltier element forthe temperature control thereof.

As a gallium-free material able to replace YAG and not suffering fromthe disadvantages of crystallogenesis and segregation of the activatingdopant, lanthanum-neodymium-magnesium aluminates (LNA) are known havingthe chemical formula La_(1-x) Nd_(x) MgAl₁₁ O₁₉ with 0<×≦1 and inparticular with x=0.1.

These aluminates form the subject matter of patents FR-A-2 448 134 andEP-A-43 776 and the publication by L. D. Schearer et al, IEEE Journal ofQuantum Electronics, vol. QE-22, No. 5, May 1986, pp 713-717 "LNA: a newCW Nd laser tunable around 1.05 and 1.08 um". The monocrystals of thesealuminates have optical properties comparable to those ofneodymium-doped YAG.

However, here again the production of these aluminates in the form oflarge monocrystals by the Czochralski method along the optical axiswhich would be of an optimum nature for the laser properties causesproblems and leads to crystals having an inadequate quality whenrequired in sizes such as are needed for power lasers.

In the field of microlasers, LNA can be optically pumped by a laserdiode, provided that a diode temperature control Peltier element isused, as for YAG.

Therefore the invention relates to a neodymium-doped gehlenite crystal,usable as a laser emitter and making it possible to obviate thedisadvantages referred to hereinbefore.

In particular, this compound can be produced in monocrystalline formwith large sizes and which suffers from neither bubbles nor defects andusing the Czochralski method. Therefore this gehlenite monocrystal canbe used in the power laser industry.

It can also be used in the field of microlasers with pumping by diodeand without using a Peltier element. Its energy efficiency and costs areconsequently well below those of known crystal monolasers.

More specifically, the invention relates to a neodymium-doped gehlenitecrystal of formula Ca_(2-x) Nd_(x) Al_(2+x) Si_(1-x) O₇ with 0<×≦1.

The crystallogenesis of this crystal according to the Czochralski methodis well controlled and can take place without there being any risk ofvolatilization of the constituents. It is performed at temperaturesbelow those used for YAG (approximately 1600° C.) and there is nosegregation of the neodymium dopant. The melting of this compound iscongruent, which gives the possibility of producing large crystalsusable as a laser emitter in a power laser.

Compared with its gallium homolog (Ca₂ Ga₂ SiO₇ :Nd), the crystalaccording to the invention has a better chemical inertia and a greaterhardness, so that it has an improved mechanical strength. It containsaluminium and not gallium, so that the cost is lower. Moreover, it musthave a higher thermal conductivity than that of the gallium homolog.Thus, in the expression of the thermal conductivity K=C.v.l, C is theheat capacity per volume unit and increases if the molecular massdecreases, v represents the velocity of the phonons in the crystal and 1is the free average path of the phonons.

Crystallographically, the lattice of the gehlenite according to theinvention is smaller than that of the gallium homologs, which leads tomodifications of the crystal field and therefore to the opticalproperties.

In particular, the crystallographic structure is more disturbed thanthat of BaLaGa O₇ :Nd. Thus, in the latter the site of Nd³⁺³ is onlysurrounded by Ga³⁺ ions, whereas in Ca₂ Al₂ SiO₇ :Nd, said seine site issurrounded both by Al³⁺ and Si⁴⁺ ions.

This disorder can be accentuated by carrying out substitutions on thethree types of cationic sites.

The compound according to the invention is intended to be used as alaser emitter emitting in the infrared, both in power lasers and inmicrolasers.

The invention also relates to a laser having a laser cavity containingas the light emitter a monocrystal, means for emplifying the light fromthe monocrystal, means for extracting the light from the laser cavityand pumping means, characterized in that the monocrystal is aneodymium-doped gehlenite of formula Ca_(2-x) Nd_(x) Al_(2+x) Si_(1-x)O₇ with 0<×≦1.

In particular, x is such that 0<×≦0.3. Advantageously, x is such that0.01≦×≦0.2 and preferably 0.025 to 0.05 and preferably x is 0.04.

The disorder of the structure leads to a widening of the emission bands.The transition ⁴ F_(3/2) →⁴ I_(11/2), which is the cause of the mainlaser emission, has an important contribution from 1.05 μm to 1.10 μm,which is remarkable if it is considered that for YAG the laserwavelength is 1.064 μm. Thus, the laser emission can be tuned as in LNA.

Thus, the crystal according to the invention has a wide wavelengthtunability range and at present it is the widest range which exists forneodymium-doped laser crystals, with the exception of glasses.

The life of the excited state is approximately 280 μs for x=0.02, forx=0.1 it is approximately 160 μs and for x=0.2 30 μs, which is a highvalue for such a doping level.

The major interest of the material according to the invention for theenvisaged laser application is that it is suitable for pumping by laserdiode. Thus, the optical absorption spectrum of Nd³⁺ in this compoundhas the same configuration of those of other Nd³⁺ activated lasermatrixes, but with the special feature here that it is the band towards800 nm (⁴ I_(9/2) →⁴ F_(5/2), ² H_(9/2)) which is the most intense,whereas in most other cases, it is the hypersensitive transition to 580nm which is the most intense. This result is particularly remarkable,because this wavelength (800 nm) exactly corresponds to the emissionrange of laser diodes.

In addition, this absorption band is very wide as a result of thedisorder between Ca²⁺ and Nd³⁺ and between Al³⁺ and Si⁴⁺ and covers thecomplete diode emission range. Moreover, even if there is wavelengthdrift in the latter case, the absorption will still be adequate.

The Nd³⁺ sites have a very low symmetry (Cs), which aids a highintensity of the absorption bands.

The values of the oscillator strength calculated for a refractive indexn=1.77 are particularly high and the highest (7.35.10⁻⁶) corresponds tothe transition ⁴ I_(9/2) →⁴ F_(5/2), ² H_(9/2) at 800 nm.

Other features and advantages of the invention can be gathered from thefollowing non-limitative description and with reference to the attacheddrawings, wherein show:

FIGS. 1 & 2 Diagrammatically the fluorescence spectrum at 300K of agehlenite crystal according to the invention.

FIG. 3 The fluorescence intensity variations as a function of theneodymium content.

FIG. 4 Part of the absorption spectrum of a crystal according to theinvention.

FIG. 5 For comparison, part of the absorption spectrum of aneodymium-doped LNA monocrystal.

FIG. 6 The laser power emitted as a function of the pumping power for agehlenite crystal with x=0.02.

FIG. 7 Diagrammatically a power laser according to the invention,optically pumped by a laser diode.

In order to produce a neodymium-doped gehlenite crystal of formula (I):

    (I) Ca.sub.2-x Nd.sub.x Al.sub.2+x Si.sub.1-x O.sub.7,

use is made of commercially available high purity powders of calciumcarbonate, neodymium and aluminium oxides, as well as silica, which areweighed out in the desired proportions. These powders are mixed forseveral hours with a mechanical stirrer and are then compressed into theshape of a cylinder. Fritting takes place for 20 hours at 1450° C. toobtain doped calcium aluminosilicate for forming the molten bath.

The starting products used are in the form of a powder with a grain sizeof 1 to 10 μm and have a purity better than 99.99%, in order to obtain amaximum efficiency for the laser emission.

The fritted mixture is then placed in an iridium crucible and brought toa temperature of 1600° C. corresponding to the melting point of themixture. Pulling takes place under an argon or nitrogen atmosphere froma nucleus having the desired orientation. Generally, pulling takes placealong axis c of the crystal. The pulling rate varies from 0.5 to 1 mmper hour and the rotation speed is approximately 40 r.p.m.

Obviously, it would also be possible to use any other crystallogenesismethod employing a molten bath such as the Bridgmann method, thefloating zone method, the Kyropoulos method or the auto-crucible method.

A perfectly monocrystalline sample can be isolated from the crystalobtained according to the Czochralski method by cleaving or cutting andpolishing, so as to obtain two strictly parallel faces. This sample canthen be placed in a laser cavity like that shown in FIG. 5 and can beoptically pumped by a laser diode emitting at approximately 800 nm.

FIGS. 1 and 2 diagrammatically show the variations of the fluorescenceintensity I_(f) (in arbitrary units) as a function of the wavelength innanometres, at 300K for a gehlenite according to the invention. FIG. 1relates to the optical transmission ⁴ F_(3/2) →⁴ I_(11/2) which is themost interesting from the laser standpoint and FIG. 2 corresponds to thetransition ⁴ F_(3/2) →⁴ I_(13/2), the latter being of interest foroptical fibre information transmission.

These curves show that the laser emission takes place on a broad band of1050 to 1100 nm for the transition ⁴ F_(3/2) →⁴ I_(11/2) and 1300 to1430 nm for the transition ⁴ F_(3/2) →⁴ I_(13/2).

Thus, the crystals according to the invention have a wide wavelengthtunability and a laser emission with a greater wavelength than that ofmost neodymium-doped laser crystals. In particular, neodymium-doped LNAemits at 1054, 1083 and 1320 nm with a wavelength tunability of a few nm(below 10 nm) and neodymium-doped YAG emits at 1064 nm with a tunabilitybelow 0.6 nm.

These fluorescence spectra were obtained using an exciting wavelength of577 nm, corresponding to the transition ⁴ I_(9/2) →⁴ G_(5/2), ² G_(7/2)in absorption of the crystal, and a composition x in neodymium of 0.02.However, it should be noted that the general configuration of thisspectrum is applicable no matter what the neodymium quantity and onlythe absorption intensity may differ slightly.

Although this is not apparent from the curves, the crystal according tothe invention also emits between 870 and 930 nm, which corresponds tothe transition ⁴ F_(3/2) →⁴ I_(9/2).

The following table I gives the life of the excited state ⁴ F_(3/2) as afunction of the Nd³⁺ ion quantity for four of the gehlenite crystalsaccording to the invention. In this table, x indicates the Nd³⁺ ionquantity, the two central columns give the short and long life of thelaser effect and the last column gives the fluorescence intensity inarbitrary units. These values were established for a 1 mm thick crystal.

Table I shows that the life decrease and the fluorescence intensity dropfor x=0.20 is linked with the auto-extinction phenomenon. Above x=0.30,the life and fluorescence intensity are inadequate for using saidcompound as a laser emitter. Compound No. 2 is that having the bestlaser properties.

FIG. 3 gives the fluorescence intensity variations If (in arbitraryunits) as a function of the doping rate x in neodymium in crystals offormula (I) . The curve of FIG. 3 was experimentally determined andclearly shows that the intensity maximum is x<0.05, particularly 0.025to 0.05 and more especially at x=0.04.

FIG. 4 shows the absorption spectrum of a gehlenite crystal according tothe invention and FIG. 5 the absorption spectrum of a neodymium-dopedLNA crystal. These curves give the optical densities (O.D.) as afunction of the wavelength in nanometres. These drawings reveal theabsorption transitions.

FIG. 4 shows that the absorption spectrum of the Nd³⁺ doped gehleniteaccording to the invention has an intense, wide absorption band around800 nm. The peaks A and B respectively correspond to absorptionwavelengths of 797.1 and 806.7 nm. The laser diodes emit in saidwavelength range.

The crystals according to the invention also have an absorption band at590 nm (hypersensitive range), peak C of the absorption curve, like mostneodymium-doped materials, but unlike in the case of otherneodymium-doped compounds, said absorption band is much less intensethan that around 800 nm. This is particularly illustrated by FIG. 5,where the absorption maximum is around 580 nm, peak D of the absorptioncurve, whereas the absorption intensity around 800 nm, peak E of curve4, is much weaker than that at 580 nm.

For information purposes, the following table II gives the oscillatorstrengths of the absorption transitions. These oscillator strengths areamong the highest observed for known neodymium-doped compounds.

As a result of the wide absorption band around 800 to 805 nm and thehigh absorption intensity, it is not necessary to use a diodetemperature control system like that using the Peltier effect. Thus,optical pumping can be ensured between 790 and 820 nm and thus coversthe wavelength drift of the diode. In addition, the crystals accordingto the invention are very suitable for laser diode optical pumping.

FIG. 6 gives the variations of the laser emission power in mW as afunction of the pumping power in mW of a gehlenite crystal ofcomposition Ca₁.98 Nd₀.02 Al₂.02 Si₀.98 O₇. These experimentallyestablished curves prove that the crystals according to the invention dohave the laser effect.

These results, given in qualitative manner, show that the tested crystalcorresponding to x=0.02 leads to a differential efficiency ofapproximately 40%. However, this crystal was not optimized. Thus, itscrystalline quality was mediocre and its doping level did not correspondto the optimum Nd³⁺ content determined on the curve of FIG. 3.

Therefore there is no doubt that with a crystal having a bettercrystalline quality and with a content x of Nd³⁺ between 0.03 and 0.05,that the laser efficiency of the material would increase considerably.

FIG. 7 diagrammatically shows a continuously operating power laser usingas the laser emitter a crystal according to the invention. This lasercomprises a laser cavity 2 containing a bar 4 of compound No. 2 placedperpendicularly to the laser longitudinal axis 3, the axis c of the barcoinciding with the laser axis 3. Laser emission is in the infrared (cf.FIG. 4).

A monochromatic light source 6, such as a laser diode or an array oflaser diodes, makes it possible to irradiate the gehlenite bar 4, via aconvergent lens 7, in order to ensure the optical pumping thereof. Adevice 5 for the circulation of distilled water around the bar 4 ensuresthe cooling thereof. However, there is no diode temperature control.

For a high laser efficiency, it is preferable for the electrical fieldof the pumping light emitted by the diode to be perpendicular to thecrystal axis c.

The laser cavity 2 is formed by a convergent lens 8 transforming thelight emitted by the gehlenite bar 4 into a parallel light beam, whichis supplied to an outlet mirror 10. After reflection on the latter, thelight beam again passes through the convergent lens 8 and the amplifyingmedium or bar 4. The amplified laser beam is then reflected by adichroic entrance mirror 12 in the vicinity of which is positioned thebar 4. This mirror 12 is transparent to the light emitted by themonochromatic source 6 and opaque to that emitted by the gehlenitemonocrystal 4.

The laser beam which has been adequately amplified in the cavity 2 isthen passed to the outside of the laser cavity, via the mirror 10, whichis partly transparent to the light emitted by the gehlenite monocrystal4.

The wavelength tunability can be obtained with the aid of a wavelengthselection system 14 placed between the convergent lens 8 and the outletmirror 10 of the laser cavity 2, of the Brewster angle prism or Lyotfilter type formed from several birefringent material plates.

In addition, a solid etalon 15 of the type formed by a plate havingparallel faces can be inserted between the convergent lens 8 and theLyot filter 14 to fix the emission wavelength.

The diode 6 has the advantage of being very small, considerably reducingthe overall dimensions of the crystal laser and it emits at a wavelengthof approximately 800 nm. The absorption spectrum of FIG. 3 reveals awide, intense absorption band around 800 to 805 nm.

The laser diodes have an excellent efficiency of approximately 50% andthe laser conversion is approximately 30 to 40%, which corresponds to alaser effect efficiency of at least 20% from the electric current.

The monocrystals according to the invention can be used in allapplications presently using a YAG laser emitter. In particular, thesemonocrystals can be used in lasers used in the cutting or marking ofmaterials, as well as for making welds.

In addition to applications of the YAG type, these oxides have specialapplications. They are particularly suitable for pumping by laser diodesand therefore to the production of miniaturized devices (militaryapplications, scientific research, medical applications). In addition,their special emission wavelengths and the tunability thereof can beadvantageously used in optical telecommunications or for thepolarization of certain atoms by optical pumping.

                  TABLE I                                                         ______________________________________                                                     Short life Long life                                                                            Fluorescent intensity                          Ex.   x      us         us     arbitrary units                                ______________________________________                                        1     0.02   --         275    101                                            2     0.04   238        284    213                                            3     0.10   142        190    307                                            4     0.20   --          30    276                                            ______________________________________                                         x represents the quantity of neodymium ions.                             

                  TABLE II                                                        ______________________________________                                        Transition* Oscillator strengths                                                                        Wavelength (nm)                                     ______________________________________                                        .sup.4 F.sub.5/2, .sup.2 H.sub.9/2                                                        7.35 × 10.sup.-6                                                                      800                                                 .sup.4 F.sub.7/2, .sup.4 S.sub.3/2                                                        6.80 × 10.sup.-6                                                                      750                                                 .sup.4 F.sub.9/2                                                                          0.47 × 10.sup.-6                                                                      700                                                 .sup.2 G.sub.7/2, .sup.4 G.sub.5/2                                                        6.29 × 10.sup.-6                                                                      590                                                                           (hypersensitive)                                    .sup.4 G.sub.7/2, .sup.2 G.sub.9/2,                                           .sup.2 K.sub.13/2                                                                         4.31 × 10.sup.-6                                                                      530                                                 .sup.4 G.sub.9/2, .sup.2 D.sub.3/2,                                           .sup.4 G.sub.11/2, .sup.2 K.sub.15/2                                                      1.94 × 10.sup.-6                                                                      480                                                 .sup.2 P.sub.1/2                                                                           0.2. × 10.sup.-6                                                                     400                                                 .sup.4 D.sub.5/2, .sup.4 D.sub.3/2                                                        7.78 × 10.sup.-6                                                                      360                                                 ______________________________________                                         *From the base level .sup.4 I.sub.9/2 of the Nd.sup.3+  ions.            

We claim:
 1. A neodymium-doped gehlenite crystal of formula Ca_(2-x)Nd_(x) Al_(2-x) Si_(1-x) O₇ with 0<×≦3, whereby said crystal has achemical inertia and hardness, so that it has an improved mechanicalstrength, and whereby said crystal has an improved thermal conductivity.2. A crystal according to claim 1, characterized in that x is chosen sothat 0.01≦×≦0.2.
 3. A crystal according to claim 1, characterized inthat x is chosen so that 0.025≦×≦0.05.
 4. A crystal according to claim1, characterized in that x is 0.04.
 5. Laser essentially having a lasercavity (2) containing as the light emitter a monocrystal (4), means (10,12) for amplifying the light from the monocrystal, means (10) forextracting the light from the laser cavity and pumping means (6, 7),characterized in that the monocrystal is a neodymium-doped gehlenite offormula Ca_(2-x) Nd_(x) Al_(2+x) Si_(1-x) O₇ with 0<×≦1.
 6. A laseraccording to claim 5, wavelength tunable in the infrared, characterizedin that it incorporates means for tuning (14).
 7. A laser according toclaim 5, characterized in that the pumping means are constituted by atleast one laser diode.
 8. A laser according to claim 7, characterized inthat the laser diode emits at approximately 800 to 805 nm.
 9. A laseraccording to any one of the claims 5 to 8, characterized in that x ischosen so that 0<×≦0.3.
 10. A laser according to any one of the claims 5to 8, characterized in that x is chosen so that 0.01≦×≦0.2.
 11. A laseraccording to claim 5, characterized in that x is chosen so that0.025≦×≦0.05.
 12. Laser according to claim 5, characterized in that x is0.04.
 13. A crystal according to claim 1, characterized in that x ischosen so that 0<×≦0.3.